The present invention relates to timing recovery in a discrete multi-tone (DMT) data communications network, particularly a network for communications between multiple devices coupled to existing wiring, for example twisted pair telephone wiring in the user""s residence.
Modern society continues to create exponentially increasing demands for digital information and the communication of such information between data devices. Local area networks use a network, cable or other media to link stations on the network for exchange of information in the form of packets of digital data. A typical local area network architecture uses a media access control (MAC) enabling network interface cards at each station to share access to the media. Conventional local area network architectures use media access controllers operating according to half-duplex or full-duplex Ethernet (ANSI/IEEE standard 802.3) protocol using a prescribed network medium, such as 10 BASE-T.
These architectures have proven quite successful in providing data communications in commercial applications. However, these common local area network architectures require installation of specialized wiring and use of specific wiring topologies. For example, the most popular network protocols, such as Ethernet, require special rules for the wiring, for example with regard to gauge and quality of wire, range of transmission and termination.
Due to the success of the Internet and the rapid decreases in the prices of personal computers and associated data equipment, a demand has arisen for data communications between a limited number of devices within relatively small premises, typically a residence or small business. While existing local area networks can serve the purpose, in such installations, the cost of installing physical network wiring satisfying the rules for the particular protocol can be prohibitively expensive.
Most existing buildings, including residences, include some existing wiring, for phones, electrical power and the like. Proposals have been made to communicate data using such existing infrastructure. This reduces the costs of wiring for the network, but the existing wiring raises a variety of issues regarding transport of high-speed digital signals.
For example, efforts are underway to develop an architecture that enables computers to be linked together using conventional twisted pair telephone lines. Such an arrangement, referred to herein as a home network environment, provides the advantage that existing telephone wiring in a home may be used to implement a home network without incurring costs for substantial new wiring installation. However, any such network must deal with issues relating to the specific nature of in-home telephone wiring, such as topology and noise. Also, operation over such a media may require sharing the media with other services without interference from or interference with the other services.
With respect to the noise issue, every device on the telephone line may be a thermal noise source, and the wiring may act much like an antenna to pick up disruptive radio signal noise. Telephone lines are inherently noisy due to spurious noise caused by electrical devices in the home, for example dimmer switches, transformers of home appliances, etc. In addition, the twisted pair telephone lines suffer from turn-on transients due to on-hook and off-hook and noise pulses from the standard telephones coupled to the lines, and electrical systems such as heating and air conditioning systems, etc.
An additional problem in telephone wiring networks is that the signal condition (i.e., shape) of a transmitted waveform depends largely on the wiring topology. Numerous branch connections in the twisted pair telephone line medium, as well as the different associated lengths of the branch connections, may cause multiple signal reflections on a transmitted network signal. Telephone wiring topology may cause the network signal from one network station to have a peak-to-peak voltage on the order of 10 to 20 millivolts, whereas network signals from another network station may have a value on the order of one to two volts. Hence, the amplitude and shape of a received pulse may be so distorted that recovery of a transmit clock or transmit data from the received pulse becomes substantially difficult.
At the same time a number of XDSL technologies are being developed and are in early stages of public network deployment, for providing substantially higher rates of data communication over twisted pair telephone wiring of the public telephone network. XDSL here is used as a generic term for a group of higher-rate digital subscriber line communication schemes capable of utilizing twisted pair wiring from an office or other terminal node of a telephone network to the subscriber premises. Examples under various stages of development include ADSL (Asymmetrical Digital Subscriber Line), HDSL (High data rate Digital Subscriber Line) and VDSL (Very high data rate Digital Subscriber Line). These DSL technologies overcome many similar problems with telephone line transport, such as noise, topology and interference with or from other services on the line.
Consider ADSL as a representative example. For an ADSL related service, the user""s telephone network carrier installs one ADSL modem unit at the network end of the user""s existing twisted-pair copper telephone wiring. Typically, this modem is installed in the serving central office or in the remote terminal of a digital loop carrier system. The user obtains a compatible ADSL modem and connects that modem to the customer premises end of the telephone wiring. The user""s computer connects to the modem. The central office modem is sometimes referred to as an ADSL Terminal Unitxe2x80x94Central Office or xe2x80x98ATU-Cxe2x80x99. The customer premises modem is sometimes referred to as an ADSL Terminal Unitxe2x80x94Remote or xe2x80x98ATU-Rxe2x80x99. The ADSL user""s normal telephone equipment also connects to the line, either directly or through a frequency combiner/splitter, which often is incorporated in the ATU-R. The normal telephone signals are split off at both ends of the line and processed in the normal manner.
For digital data communication purposes, the ATU-C and ATU-R modem units create at least two logical channels in the frequency spectrum above that used for the normal telephone traffic. One of these channels is a medium speed duplex channel; the other is a high-speed downstream only channel. Two techniques are under development for dividing the usable bandwidth of the telephone line to provide these channels. One approach uses Echo Cancellation. Currently, the most common approach is to divide the usable bandwidth of a twisted wire pair telephone line by frequency, that is to say by Frequency Division Multiplexing (FDM).
FDM uses one frequency band for upstream data and another frequency band for downstream data. The downstream path is then divided by time division multiplexing into one or more high-speed channels and one or more low speed channels. The upstream path also may be time-division multiplexed into corresponding low speed channels.
The most common form of the FDM data transport for DSL services utilizes discrete multi-tone (DMT) technology. A DMT signal is basically the sum of N independently QAM modulated signals, each carried over a distinct carrier frequency channel. The frequency separation of each carrier is 4.3125 kHz with a total number of 256 carriers or tones (ANSI). An asymmetrical implementation of this 256 tone-carrier DMT coding scheme might use tones 32-255 to provide a downstream channel of approximately 1 MHz analog bandwidth. In such an implementation, tones 8-31 are used as carriers to provide an upstream channel of approximately 100 kHz analog bandwidth. Each tone is quadrature amplitude modulated (QAM) to carry up to 15 bits of data on each cycle of the tone waveform.
The existing DSL systems provide effective high-speed data communications over twisted pair wiring between customer premises and corresponding network-side units, for example located at a central office of the public telephone network. The DSL modem units overcome many of the problems involved in data communication over twisted pair wiring. However, for a number of reasons, the existing DSL units are not suitable to providing local area network type communications within a customer""s premises. For example, existing ADSL units are designed for point-to-point communication. That is to say, one ATU-R at the residence communicates with one ATU-C unit on the network end of the customer""s copper line. There is no way to use the units for multi-point communications. Also, the existing ADSL modems tend to be quite complex, and therefore are too expensive for in-home communications between multiple data devices of one customer.
A need therefore still exists for techniques to adapt DMT type DSL communications for use over existing in-home wiring. The adaptations should enable multi-point communications. Also, many of the problems overcome by complex methodologies in ADSL communications need corresponding simpler, more cost effective solutions for in-home networking.
For example, digital signal sampling and processing to decode DMT data signals requires accurate timing between the transmitter and the receiver. In existing ADSL communications, one of the tone frequency channels is used as a pilot tone channel. DMT demodulation and decoding for all other channels is based on recovery of timing information from the pilot tone. This eliminates transmission of data over the channel dedicated to the pilot tone. Also, coordination of reception of all of the other channels to the timing from the one pilot tone channel is extremely complex. A need therefore exists for a simpler form of timing recovery, particularly one that is readily adaptable to a multi-point network using existing wiring such as twisted pair telephone wiring on a user""s premises
In a multi-point, random access communication application, the timing problem becomes more acute. Unlike the point-to-point DSL implementations where communications are always on-going and enable virtually continuous synchronization between transmitter and receiver, the random access type devices only send when they have data to send. There is no on-going synchronization. As a result, there is a need to initiate timing during a communication to determine when and where in the received signal the receiver should look to demodulate the various elements of received information.
The present invention overcomes the noted problems involved in data networking and satisfies the above stated needs by providing an improved timing recovery technique, at the physical layer, for use in a multi-point DMT communication system. Specifically, at the start of a random access data communication, the transmitter sends a timing mark, preferably in the form of a number of cycles of a defined periodic or cyclical signal. Subsequent sampling operations of the DMT receiver, particularly demodulation of DMT symbols, uses sampling periods at predetermined times following the timing mark. For example, in the preferred embodiments, each packet of data begins with a sinusoidal timing mark of a predetermined frequency, before there is ever any transmission of multiplexed tone signals for DMT symbols. The receiver bases all timing, for processing samples of the symbols containing the information for the packet of data, on counting numbers of sampling intervals following the end of the timing mark.
In the preferred embodiment, a transmitter seeking access to the link initially sends a timing mark followed by an identification and control signal, used for collision detection. If there is no collision, the transmission continues with transmission of DMT symbols. All timing of decoding of the signals, such as the ID signals and the subsequent DMT symbols, is based on counting signal sampling intervals following the detected timing mark. The preferred embodiment utilizes differential modulation of the tones within the DMT symbol. Each packet may include a reference symbol to enable demodulation of the first DMT data symbol. Also, the preferred embodiment of the DMT symbol includes a prefix code followed by the actual DMT waveform carrying the modulated information. The prefix code corresponds to a fixed number of samples of time-domain data, such as a copy of the last thirty-two samples used to generate the actual DMT waveform of the particular symbol.
The maximum benefit of this timing recovery scheme is achieved when used with the cyclic prefix code of samples from the end of the DMT waveform and with differential encoding of the data. Timing errors resulting in processing of several samples of the prefix will produce only a phase error in the DMT processing, however, because of the use with differential decoding there will be no errors in the decoded data.
The inventive timing recovery technique is particularly simple. Also, once the time mark is detected, there is no need to change any further timing operations. There is never any need to adjust the sampling clock.
Aspects of the invention relate to methods of transmission and reception as well as DMT transmitters and receivers implementing the concepts of the invention. Another aspect of the invention relates to a carrier signal bearing the timing mark and one or more DMT symbols in a form consistent with the invention.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.